The invention relates to a control system and method for selecting one of first and second cylinder groups of an internal combustion engine when a frequency of air/fuel ratio oscillations in the selected cylinder group is to be adjusted.
Known internal combustion engines generally have first and second cylinder groups that are connected to either straight pipe exhaust systems or Y-pipe exhaust systems. Both types of exhaust systems further utilize catalytic converters to reduce automotive emissions.
Known engine control systems utilize a measurement of exhaust gases from the first and second cylinder groups to maintain a predetermined air/fuel ratio in each of the first and second cylinder groups. Generally, the predetermined air/fuel ratio is stoichiometric. Further, when two state exhaust gas oxygen sensors are utilized, a typical result is a fluctuation, or oscillation, of the exhaust air/fuel ratio about the predetermined air/fuel ratio.
With engines having Y-pipe exhaust systems, known control systems have attempted to phase shift air/fuel ratio oscillations in a first cylinder group out of phase with air/fuel ratio oscillations in a second cylinder group, to reduce emissions. The inventors herein have recognized that the first and second cylinder groups may have first and second air/fuel ratio oscillations, respectively, that are operating at substantially different frequencies. Thus, when this occurs, merely phase shifting one of the air/fuel ratio oscillations with respect to the other, may not result in the desired phase difference being reached and/or maintained. Thus, inventors herein have recognized that to maintain a desired phase offset between the first and second air/fuel ratio oscillations, the first and second frequencies should be matched.
It is also known that increasing a peak-to-peak amplitude of air/fuel ratio oscillations in a cylinder group results in increased engine torque fluctuations and associated engine vibrations. Known control systems attempt to maintain a predetermined peak-to-peak amplitude that provides a predetermined allowable amount of engine torque fluctuations while enabling downstream catalytic converters to reduce engine emissions.
The inventors herein have recognized that the first and second frequencies of the first and second air/fuel ratio oscillations, respectively, can be matched by (i) either decreasing the faster frequency or (ii) increasing the slower frequency. However, the inventors have recognized that attempting to increase the slower frequency of air/fuel ratio oscillations results in an increased peak-to-peak amplitude of the respective oscillations. Thus, by attempting to increase the slower frequency of air/fuel ratio oscillations, engine torque fluctuations are undesirably increased.
The inventors herein have recognized that the faster frequency of air/fuel ratio oscillations can be decreased while maintaining the desired peak-to-peak amplitude of the respective oscillations. Thus, the faster frequency can be decreased to match the slower frequency of air/fuel ratio oscillations, without generally increasing engine torque fluctuations. Thus, the inventors have recognized that the cylinder group having the faster frequency of air/fuel ratio oscillations should be selected when adjusting a frequency of air/fuel ratio oscillations to minimize engine torque fluctuations.
The invention relates to a control system and method for selecting one of first and second cylinder groups of an internal combustion engine when a frequency of air/fuel ratio oscillations in the selected cylinder group is to be adjusted.
The method for selecting one of first and second cylinder groups includes determining which of first and second frequencies of air/fuel ratio oscillations in the first and second cylinder groups, respectively, has a greater frequency. Further, the method includes selecting one of the first and second cylinder groups having the greater frequency of air/fuel ratio oscillations wherein the selection is made to reduce engine torque fluctuations while changing a frequency of air/fuel ratio oscillations in the selected cylinder group.
A control system for selecting one of first and second cylinder groups of an internal combustion engine is provided. The control system includes a first device for determining a first frequency of air/fuel ratio oscillations in the first cylinder group. The control system further includes a second device for determining a second frequency of air/fuel ratio oscillations in the second cylinder group. Finally, the control system includes a controller operatively connected to the first and second devices. The controller is configured to determine which of the first and second frequencies has a greater frequency. The controller is further configured to select one of the first and second cylinder groups having the greater frequency of air/fuel ratio oscillations.
The inventive control system and method provides several advantages over known control systems and methods. In particular, the system and method selects a cylinder group that minimizes engine torque fluctuations and vibrations when a frequency of air/fuel ratio oscillations therein is adjusted to match another frequency. Further, the system and method selects a cylinder group that minimizes engine emissions when a frequency of air/fuel ratio oscillations therein is adjusted to match another frequency.